1. Field of the Invention
The invention is related to congruently melting lanthanum indium gallium garnets, which are suitable as substrates in magneto-optic and magnetic thin film applications.
2. Description of the Prior Art
The garnet family contains hundreds of compounds with the same crystal structure (cubic; space group Ia3d), and the number of new compounds being discovered is constantly increasing. The garnet structure with its three cation sublattices capable of accepting ions in different coordination polyhedra (dodecahedral, octahedral and tetrahedral) offers numerous ways of varying the physical properties of the resulting compounds. Substitutions are governed by strict rules based on the crystal chemistry of ternary and more complex systems. Garnet materials have found suitable use in applications such as optical materials, magneto-optic materials, magnetic bubble materials and substrate materials.
Of optical materials, only neodymium-doped yttrium aluminium garnet (YAG:Nd) has attained substantial commercial use as a laser. Typically, this garnet is employed in bulk single crystal form, and does not usually involve substrate material.
With regard to magneto-optic materials, applications of the Faraday rotation effect have resulted in efforts to prepare garnets containing praseodymium, neodymium and bismuth, since these ions display high rotation effects in garnet materials. However, these ions also increase the lattice parameter of such garnets. Because these materials are in the form of thin films expitaxially grown on a substrate, compromises must be made between the amount of ions present which contribute to a high Faraday rotation and the lattice parameter of the substrate in order to achieve as close a match as possible in lattice parameters of the thin film and substrate. Otherwise, the resulting mismatch in lattice parameters results in undue strain, which detrimentally affects the desired properties.
With regard to magnetic bubble domain materials, a large number of such materials have been developed, but materials evidencing such desirable properties as high bubble mobility and small bubble diameter (&lt;1.0 .mu.m) often possess lattice parameters that are larger than those of commercially available substrates. Again, a compromise must be made between the properties of such materials and the lattice parameter of the substrate in order to achieve lattice matching of the thin film and substrate. Typically, it is desired to match lattices within at least a fraction of 1%.
With regard to substrate materials, utilization of any new garnet material and the expansion of garnet thin-film technology depends on the availability of substrate material with a range of lattice parameters. Because of recent work on high Faraday rotation materials and on new magnetic bubble domain materials, there is increased effort on the crystal chemistry and crystal growth of garnets that can meet present and future requirements. In order to provide magneto-optic materials and magnetic bubble domain materials having optimum properties without compromising lattice parameters, new substrate materials having larger lattice parameters than heretofore available are needed. At present, the only material available in large scale is gadolinium gallium garnet (GGG; Gd.sub.3 Ga.sub.5 O.sub.12), which has a lattice parameter a.sub.o of 12.384 Angstroms (A). Mixtures of gadolinium gallium garnet and neodymium gallium garnet (NdGG; Nd.sub.3 Ga.sub.5 O.sub.12 ; a.sub.o =12.504 A) have been utilized as substrate material to achieve lattice parameters which range linearly with composition from 12.384 to 12.504 A. However, materials suitable for substrates having lattice parameters up to about 13 A are required in order to accommodate new magnetic materials, such as new magneto-optic and magnetic bubble materials, having such larger lattice parameters.
Further, such materials must be capable of forming single crystals by melt-growth techniques so as to yield single crystals of a size suitable for substrate fabrication. In order to grow single crystals of a particular composition from the melt, that composition must exhibit congruent melting.
Garnets specifically including indium (In.sup.3+) have been previously reported. For example, the preparation of polycrystalline Y.sub.3 In.sub.2 Ga.sub.3 O.sub.12, with a.sub.o =12.548 A, was reported in the Journal of Applied Crystallography, Vol. 6, pp. 416-7 (1973).
The use of Sm.sub.3 Ga.sub.5-x In.sub.x O.sub.12 to support bismuth-substituted Gd.sub.3 Fe.sub.5 O.sub.12 magneto-optic films was reported in AIP Conference Proceedings, No. 18, Part 2, pp. 944-948 (1974). However, no values of "x" or a.sub.o were reported.
A description of small (about 4 mm), crystals having the composition La.sub.2.97 In.sub.1.94 Ga.sub.3.09 O.sub.12 appeared in Chemical Abstracts, Vol. 77, p. 429, No. 120353d (1972). Although the lattice parameter was given as 12.915 A, small crystals are not suitable for substrate use, which requires boules of at least 0.5 inch diameter and 2 inch length. Further, the composition has been found to be incongruently melting, which renders it unsuitable for growing large boules by melt-growth techniques, such as Czochralski or Bridgman-Stockbarger, for substrate applications.
Thus, prior art garnets are not available evidencing both a large lattice parameter (e.g., about 13 A) and congruent melting.
In addition to the desired increase in lattice parameters indicated above, suitable substrate materials must be non-ferromagnetic, in order to avoid magnetic coupling effects which would deleteriously affect the supported thin film. Also, the substrate material must be chemically stable.